1. Field of the Invention
The present invention relates to a pneumatic tire in which a sipe is formed in a land portion of a tread surface, and is particularly useful as a studless tire.
2. Description of the Related Art
Conventionally, in the studless tire, a cut called as the sipe is formed in the land portion of a block, a rib or the like, and a traveling performance on an ice road surface having a low friction coefficient is increased by an edge effect and a drainage effect generated by the sipe. As the sipe mentioned above, there have been put to a practical use a linear sipe extending linearly along a longitudinal direction, a waveform sipe extending like a waveform and the like.
In this case, since a rubber is incompressible, a motion of the tread surface being in contact with the road surface becomes great in the case that a load is applied to the tire, so that an opening portion of the sipe tends to be closed. Particularly, since the tread surface tends to be moved due to its slip, the sipe is easily closed and the opening portion is narrowed, on the ice road surface having the low friction coefficient, there is a case that the edge effect and the drainage effect to be essentially achieved by the sipe are lowered.
Accordingly, a tire described in Japanese Unexamined Patent Publication No. 2007-8303 is structured such as to make a tread rubber be easily deflected in a depth direction by providing a plurality of wide portions in a sipe, and absorb a deformation by the wide portions so as to inhibit an opening portion from being narrowed. Further, a tire described in Japanese Unexamined Patent Publication No. 2005-247105 is structured such that an opening portion is not completely closed in a wide portion at a time when a sipe is closed, by providing the wide portion extending in a depth direction from a tread surface so as to terminate without reaching a bottom portion of the sipe.
On the other hand, if a wear makes progress and a height of a land portion is reduced, a rigidity of the land portion is improved and a collapse of the land portion becomes small, so that there is a tendency that the edge effect generated by the sipe is lowered. As a result, there is a case that a braking performance (an ice braking performance) or the like on an ice road surface is deteriorated in accordance with a progress of the wear. On the contrary, in the case that a sipe width is previously set larger, the land portion excessively collapses in an initial stage of the wear such as a state in which tires are new, and the edge effect is further lowered.
With respect to this problem, it becomes important to make the rigidity of the land portion proper in correspondence to a stage of progress of the wear so as to secure the collapse of the land portion. However, the sipe structures described in Japanese Unexamined Patent Publication Nos. 2007-8303 and 2005-247105 do not disclose a solving means thereof. Further, Japanese Unexamined Patent Publication No. 2006-36102 proposes a structure securing the collapse of the land portion after a medium stage of the wear by providing a wide portion reaching a bottom portion of the sipe from a position coming in a depth direction from the tread surface. However, it has been known that there is a possibility of further improving a performance.